CHAP. VII.] DEEP-SEA TEMPERATURES. 285 



liarities in temperature, owing to the mixture of 

 hot and cold currents of air ; but in the main, iso- 

 thermal lines, that is to say, lines drawn through 

 places having the same mean temperature, would 

 coincide with parallels of latitude. A glance at any 

 isothermal chart, whether for the whole year, for 

 summer, for winter, or for a single month, will show 

 that this is far from being the case. The lines of 

 equal temperature deviate everywhere, and often 

 most widely, from their normal parallelism with the 

 parallels of latitude and with each other. A glance 

 at the same chart will also show, that while there 

 is an attempt, as it were, on the part of the iso- 

 thermal lines to maintain their normal direction 

 through the centre of great continents, the most 

 marked curves, indicating the widest extensions of 

 uniform conditions of temperature, are where there 

 is a wide stretch of open sea extending through 

 many degrees of latitude, and consequently includ- 

 ing very different climatal conditions. 



The lands bordering upon the ocean partake in 

 this general diffusion of heat and amelioration of 

 climate, and hence we have the difference between 

 continental and insular climates — tlie former giving 

 extremes of summer heat and winter cold, and the 

 latter a much more uniform temperature, somewhat 

 below the normal temperature within the tropics, 

 and usually greatly above it beyond their limits. 



The islands of Ireland and Great Britain and the 

 west coast of the Scandinavian peninsula are in- 

 volved in the most extreme system of abnormal 

 curves which we have in any of the ocean basins : 

 and to this peculiarity in the distribution of tern- 



